Bugfix for HEAD request test; they should return empty content.

The conversion from Dancer2::Test to Plack::Test introduced a bug in the test
for HEAD requests; it was (erroneously) modified to expect content in the
response whereas the content should be empty for a HEAD request.
Use Plack middleware for HEAD request content removal.

The logic in the around => dispatch modifier required a request to be passed in
before it would remove content from the response of a HEAD request. All the
current Servers only supply the psgi env and no content removal was occuring.
(Plack::Middleware::Head is generic middleware that handles exactly this case)

Note that previously Dancer2::Test was supplying both the env and a request
causing the test to pass. Another subtle win for moving to Plack::Test.
Reflect change.